Magnetic resonance imaging (MRI) is a technique that obtains images by placing the human body inside a large magnet tube that generates a magnetic field, resonating hydrogen atom nucleus in the body by generating high frequency waves, measuring the differences in signals from various tissues of the body, and reconstituting the signals. Namely, it is to obtain images by giving high frequency waves to the body in the large magnet tube to generate a signal like echo from the human body and converting the signal into digital information.
Magnetic resonance imaging (MRI) has an important advantage in that it is actually harmless to the human body, because it uses high frequency waves that are non-ionizing radiations, unlike simple X-ray imaging or computer tomography (CT). Also, because MRI uses a magnetic field harmless to the human body and high frequency waves that are non-ionizing radiations, it has an excellent contrast for soft tissues without a contrast agent, compared to CT and enables to obtain information on the biochemical characteristics of tissues containing hydrogen atom nuclei. Although MRI is similar to CT in that it shows cross-sectional images, MRI also has an advantage in that images of the human body in a desired direction among a horizontal axis direction, a longitudinal axis direction, a diagonal direction and the like can be easily obtained without changing the patient's posture, whereas, in CT, images of the human body in a horizontal axis direction are mainly obtained.
However, because the water content of the human body is 70% or more and the difference in the concentration between portions of the human body is also insignificant, the sensitivity of images in a process of imaging MRI signals is low, and thus MRI shows limitations in distinguishing between normal tissue and abnormal tissue in the same organ and has a shortcoming in that early diagnosis of disease is not easy.
MRI contrast agents are auxiliary materials that increase the sensitivity of magnetic resonance imaging by maximizing the difference in signals and have been widely used in clinical applications. MRI contrast agents are largely classified into T1 and T2 contrast agents. T1 has the effect of causing a labeled portion to appear lighter than the surrounding tissue, whereas T2 has the effect of causing a labeled portion darker than the surrounding tissue. T1 contrast agents that are currently typically used in clinical applications include gadolinium ions (Gd2+), and T2 contrast agents include iron oxide nanoparticles.
In recent years, studies using an MRI contrast agent have been actively conducted in which a specific site of the human body is labeled and the developmental process thereof is observed with time or in which a specific cell or foreign material is labeled and injected into the human body, and then the migration pathway thereof is investigated. This provides clues to enable to establish the causes of diseases, which have not been established, or to establish processes of treating diseases using novel drugs or metabolic procedures.
Typical contrast agents based on iron oxide nanoparticles, which are currently used in clinical applications, are obtained by dissolving ferrous chloride hydrate (FeCl2.H2O) and ferric chloride hydrate (FeCl3.H2O) in an aqueous solution and reducing iron ions by inducing a basic solution. Dextran that is a kind of polysaccharide is added to form a coating film on the surface of the iron oxide nanoparticles in order to maintain the stability of the particles. The nanoparticles synthesized by the above manner have an advantage of excellent biocompatibility, but have a low effect on the amplification of magnetic resonance signals, because of their small particle size and low surface crystallinity. In addition, the nanoparticles have a shortcoming in that they do not have a functional group capable of chemically binding to a target ligand for selectively labeling a specific site in the human body. As a result, it is difficult to form target-specific particles, and the contrast effect of the particles is not sufficient to sense a cellular or molecular target.
Recently, as an alternative material for iron oxide nanoparticles synthesized by the aqueous solution method, iron oxide nanoparticles synthesized at high temperature in an organic solvent have been receiving attention. In this synthesis method, the formation of iron ion particles at high temperature occurs slowly, and thus the particles formed have a very uniform size and shape and a very high surface crystallinity. In addition, there is an advantage in that the size of the nanoparticles can be easily controlled by changing either the amount of a surfactant that is added to stabilize the particles during synthesis or the reaction temperature. In recent years, nanoparticles having a significantly improved signal amplification effect as a result of adding a transition metal such as manganese or cobalt to iron oxide particles have been reported. However, because these nanoparticles are synthesized in an organic solvent, they are hydrophobic in nature. For this reason, in order to inject these nanoparticles into the human body, the phase of the nanoparticles should necessarily be transferred to an aqueous phase. For this purpose, methods are used in which the surface of hydrophobic nanoparticles is coated with an amphipathic phospholipid or a biodegradable polymer material having a hydrophobic group chemically introduced therein. However, such methods have many problems in terms of the stability and biocompatibility of the particles.
US Patent Publication No. 20090180966 discloses a method for preparing nanoparticles comprising anions and cations that form ionic bonds with each other, the method comprising complexing anionic and cationic polymers with metal ions serving as contrast agents for magnetic resonance imaging. However, the nanoparticles prepared by this method have a disadvantage in that the sensitivity of magnetic resonance imaging signals is significantly lower than that of signals obtained using nanoparticle contrast agents for magnetic resonance imaging. In addition, there is a disadvantage in that the metal ions are actually difficult to use as contrast agents for magnetic resonance imaging, because the toxicity thereof is very strong.
Accordingly, the present inventors have made extensive efforts to prepare a nano-contrast agent for magnetic resonance imaging using the ionic self-assembly properties of poly-gamma-glutamic acid (γPGA) and chitosan, which are biocompatible polymer materials, and as a result, have found that, when iron oxide-based nanoparticles are encapsulated in a complex of poly-gamma-glutamic acid (γPGA) and a chitosan polymer, they effectively increase magnetic resonance signals to increase the label sensitivity of a specific cell or organ in the human body, thereby completing the present invention.